This invention relates to a method of treating particulate uranium oxide compositions containing ammonium diuranate precursors such as uranyl fluoride and uranyl nitrate, and in greater detail in a particularly preferred practice relates to treating a particulate uranium oxide composition containing a mixture or blend of uranium tritaoctoxide (U.sub.3 O.sub.8) and uranyl fluoride (UO.sub.2 F.sub.2) and other fluoride compound impurities including uranium tetrafluoride (UF.sub.4) in which the percentage of fluoride varies from a low of about 0.7 percent or less to about 7 percent or more on a weight percent basis.
Ammonium diuranate as used herein means the term normally used in commercial practice and approaching the specific composition represented by the formula (NH.sub.4).sub.2 U.sub.2 O.sub.7.
Oxide products of uranium have various utilities including a preferred utility as nuclear fuels for nuclear reactors. The performance of the fuel, traditionally in the form of enriched uranium dioxide structures clad in a metal container, is crucial to the proper operation of the nuclear reactor and is related to the particular structure of the uranium dioxide. Nuclear power generation has imposed severe requirements on the performance of such fuels in nuclear reactors, and the performance is strongly influenced by the grain size and density of the fuel. It has been demonstrated that fine grain uranium dioxide structures are more subject to creep than large grain uranium dioxide structures. It has also been discovered that the density of the uranium dioxide is a very important physical property influencing the performance of the fuel. In order to fabricate uranium dioxide economically and reproducibly, the starting ceramic powder should be capable of compaction to give a compact of desired density and adequate green strength with a low impurity level.
The enrichment of uranium customarily is accomplished through use of the compound uranium hexafluoride (UF.sub.6). A process is therefore required for converting the enriched uranium hexafluoride into enriched uranium dioxide in a form which can be readily fabricated to fuel structures having a low fluoride content and a desired density and grain size.
One current process for converting uranium hexafluoride to an oxide product of uranium, usually uranium dioxide, employs hydrolysis of uranium hexafluoride to form a solution of uranyl fluoride and hydrogen fluoride from which ammonium diuranate is precipitated by the addition of ammonia. After filtration, the ammonium diuranate containing undesirable fluoride impurities is dissolved in nitric acid. Fluoride decontamination of the resulting uranyl nitrate solution is accomplished by solvent extraction. From the resulting purified uranyl nitrate solution, ammonium diuranate is reprecipitated and then calcined to U.sub.3 O.sub.8 which in turn is reduced to uranium dioxide with heated hydrogen.
Attempts have been made to replace this involved, expensive ammonium diuranate conversion process by gas phase reaction of uranium hexafluoride. A very successful method is described in U.S. Pat. No. 3,796,672 entitled "Process for Producing Uranium Dioxide Rich Compositions from Uranium Hexafluoride", and the disclosure of this patent is hereby incorporated by reference. This patent is in the names of W. R. DeHollander and A. G. Dada and is assigned to the same assignee as the present invention. This patent discloses a process for the conversion of gaseous uranium hexafluoride to a uranium dioxide rich composition in the presence of an active flame in a reaction zone. A gaseous reactant comprising a reducing gas and a gaseous reactant comprising a mixture of uranium hexafluoride and an oxygen-containing carrier gas are separately introduced to the reaction zone. The reactants are temporarily separated by a shielding gas during the introduction to the reaction zone. The shielding gas delays the initiation of the reaction between the gaseous reactants until sufficient cross diffusion and mixing of the reactants occurs. The process of U.S. Pat. No. 3,796,672 yields a uranium dioxide-rich composition having particularly desirable properties and a gaseous atmosphere rich in reducing gas such as hydrogen.
Since it is known that certain gaseous mixtures of a reducing gas such as hydrogen and air can be readily combustible and potentially explosive, it has been found desirable to oxidize the hydrogen in such a gaseous atmosphere following the UF.sub.6 to UO.sub.2 conversion.
U.S. Pat. No. 3,790,493 entitled "Post Oxidation Process for Uranium Dioxide Rich Compositions" covers an improved process in which an oxygen-containing gas is introduced as a third gaseous reactant at a time when the uranium hexafluoride conversion to the uranium dioxide rich composition is substantially complete. The uranium dioxide rich composition is thereby oxidized to a higher oxide of uranium and the residual reducing gas is oxidized to form water vapor. This patent in the names of Abdul G. Dada, W. R. DeHollander and Robert J. Sloat is assigned to the same assignee as the present invention, and the disclosure of this patent is hereby incorporated by reference.
Another very successful method of replacing the ammonium diuranate conversion process by gas phase reaction of uranium hexafluoride is described in copending U.S. patent application Ser. No. 663,274, now U.S. Pat. No. 4,090,976, entitled "Process for Producing Uranium Oxide Rich Compositions from Uranium Hexafluoride", and the disclosure of this application is hereby incorporated by reference. This patent application was filed Mar. 3, 1976 in the names of W. R. DeHollander and C. P. Fenimore and is assigned to the same assignee as the present invention. This process accomplishes the conversion of gaseous uranium hexafluoride to a uranium oxide-rich composition in the presence of an active flame in a reaction zone. A first gaseous reactant comprising a mixture of uranium hexafluoride and a reducing carrier gas and a second gaseous reactant comprising an oxygen-containing gas are separately introduced to the reaction zone. The reactants are temporarily separated by a shielding gas during their introduction to the reaction zone. The shielding gas delays the initiation of the reaction between the gaseous reactants until sufficient cross diffusion and mixing of the reactants occurs. The flame in the reaction zone is maintained away from contact with the inlet through which the mixture is introduced to the reaction zone.
The processes of U.S. Pat. Nos. 3,796,672 and 3,790,493 and U.S. patent application Ser. No. 663,274, now U.S. Pat. No. 4,090,976, yield a uranium oxide composition containing fluoride impurities, and this composition is a very desirable, active, highly sinterable uranium oxide in the form of a powder. For some applications the powder is very desirable since compacts of such powder sinter to a very high density, typically close to 99 percent or higher of theoretical density. However for other applications it is desirable to reduce the density of sintered compacts of the powder to the range of about 92 to about 97 percent of theoretical density. Further, it is desirable to provide even greater strength to the powder compacts prior to sintering so that they retain their configuration through sintering.